Hello.I am having a problem in understanding the Collector Characteristic Curves of a transistor.
In the figure above,why is Ib=0 till the cut-off Region?I have read my text book but I do not understand the relationship of Ib with this graph.Please help me out.Thank you

Saturation as far as voltage is concerned, is when the collector voltage is less than the base voltage, and so now the VCB is forward biased rather than reversed biased.

So looking at the graph if you apply 120uA. of base current your going to have a lot of collector current as the graph shows, however if the voltage at the collector is small VCE around 1v. as shown then the base is more positive than the collector meaning the transistor is saturated, but if you bring the collector voltage VCE higher with that amount of base current, now the collector is getting into the region of making the VCB to be reversed biased as it should be to work as an amplifier.

So the larger the base current the more VCE needs to be to keep the transistor in its linear region of amplification, characteristics.